Cellulose-based foodstuff casings prepared by the amine oxide process

ABSTRACT

The invention concerns flat or tubular cellulose-based foodstuff casings which are prepared by extruding (“spinning”) cellulose dissolved in N-methyl-morpholine-N-oxide by means of a nozzle with an annular gap. After extrusion, the casings are stretched transversely in ambient air by blow moulding, and are then treated with an NMMO-containing aqueous spinning bath. The tubular casings are particularly suitable as sausage casings. Cut open, they can also be used as flat foils.

The invention relates to flat or tubular cellulose-based food casingswhich are produced by extruding (“spinning”) cellulose dissolved inN-methylmorpholine N-oxide. The casings are particularly suitable assausage casings.

Cellulose is insoluble in the usual solvents. It does not have a meltingpoint or melting range and cannot therefore be melt-processed either.Therefore, it is usually chemically modified for producing food casings.However, these processes are associated with a breakdown of thecellulose, i.e. the mean degree of polymerization of the cellulosebecomes lower. In addition, the processes are highly technically complexand correspondingly expensive.

Currently, the viscous process is preferred. In this process, thecellulose is reacted with sodium hydroxide solution and then with carbondisulfide. This produces a yellow-orange-colored cellulose xanthogenatesolution which is extruded through a spinneret. The cellulose is thenregenerated using precipitation and washing baths. A variety ofapparatuses have had to be developed for this for cleaning up exhaustair and wastewater.

As early as 1936 it was discovered that cellulose is soluble in oxidesof tertiary amines (DE 713 486); however, this discovery was not pursuedfurther until 30 years later. In the course of this, N-methylmorpholineN-oxide (NMMO) was identified as the most suitable solvent. Thecellulose dissolves therein without being chemically modified. Nobreakdown of the cellulose chains takes place. Preparation of thecorresponding spinning solutions has also been disclosed (DD 218 104; DD298 789; U.S. Pat. Nos. 4,145,532; 4,196,282; 4,255,300). Yarns may beproduced from the solutions by extrusion into a spinning bath (DE-A 4409 609; U.S. Pat. No. 5 417 909). WO 95/07811 (=CA 2 149 218) alsodiscloses a process for producing tubular cellulose films by the amineoxide process. A characteristic of this process is cooling the extrudedfilm by cooling gas immediately below the annular gap of the extrusiondie. According to EP-A 662 283, the extruded tubular film is cooledinternally using liquid.

Recovery and purification of the NMMOs are described in DD 274 435.Since the cellulose is not chemically modified in the process, lessequipment is required. In the amine oxide process, no gaseous or aqueouswaste products are produced, so that there are no problems with respectto the exhaust air or the wastewater. It is therefore achievingincreasing importance.

EP-A 0 686 712 describes the production of flexible cellulose fibers bythe N-methylmorpholine N-oxide (NMMO) spinning process. In this process,a cellulose solution in aqueous NMMO is forced through a spinneret,conducted via an air section into an NMMO-containing aqueousprecipitation bath and then washed, post-treated and dried.

According to WO 93/13670, a seamless tubular food casing is produced byextruding a solution of cellulose in NMMO/water using a specialextrusion die. An air section is situated between extrusion die andprecipitation bath. A characteristic of this process is a speciallyshaped hollow mandrel through which the precipitation liquid can alsocirculate in the interior of the tube. In the air section, the interiorof the extruded tube is virtually completely filled by a hollow mandreland precipitation liquid. The film is not stretched transversely in thecourse of this.

WO 95/35340 describes a process for producing blown cellulose films inwhich an underivatized cellulose dissolved in NMMO is used.

However, the amine oxide process also has disadvantages. Theunderivatized cellulose molecules are already preorientated in the NMMOsolution and are substantially more tightly packed than is the case withchemically modified (“derivatized”) molecules. On extrusion, theorientation in the longitudinal direction is still more pronounced. Theyarns thus produced therefore exhibit a high strength in thelongitudinal direction, but only low strength in the transversedirection. They have a strong tendency to split on being mechanicallystressed in the wet state. Films or other shaped bodies, which must beable to be loaded in the longitudinal and transverse direction, may thusscarcely be produced by this method.

The object was therefore to modify the amine oxide process in such amanner that sufficiently load-bearing films or shaped bodies, inparticular tubular food casings, can be produced. The process shouldsucceed in this case with as few steps as possible, and should remaininexpensive and environmentally compatible.

The object can be achieved if the wet treatment is combined with a blowmolding. The present invention thus relates to a seamless tubularcellulose-based film, which is obtainable by extruding a cellulose-,N-methyl-morpholine N-oxide- and water-containing spinning solutionthrough an annular die and treating the tubular film in anN-methylmorpholine N-oxide-containing aqueous spinning bath, wherein thespinning solution comprises 0.2 to 50% by weight, based on the weight ofthe cellulose, of modifying compounds which increase the suppleness,strength, clipping stability and shear stability of the tubular casing.

The spinning solution preferably comprises 7 to 15% by weight,particularly preferably 9 to 12% by weight, cellulose, in each casebased on the total weight of the spinning solution. The mean degree ofpolymerization of the cellulose in this case is preferably 300 to 700,particularly preferably 400 to 650. As solvent, the spinning solutionpreferably comprises 90.5 to 92.5% by weight NMMO and 9.5 to 7.5% byweight water. The parameters mentioned in this paragraph, together withthe temperature, essentially determine the viscosity and fluid behaviorof the spinning solution.

Processes for preparing the spinning solution are generally familiar tothose skilled in the art. Customarily, cellulose is mashed in a 60%strength by weight aqueous NMMO solution at room temperature. Thecellulose usually originates from wood or cotton. As the temperatureincreases, water is then distilled off in a heated stirred tank underreduced pressure until the residue consists of cellulose and NMMOmonohydrate. This is the case at an NMMO content of 87.7% by weight,based on the total weight of NMMO and water. The ratio of NMMO to watermay be readily determined by the refractive index. In the NMMOmonohydrate, the cellulose dissolved completely at a temperature of 85to 95° C. with intensive stirring. The refractive index of the solutionis 1.4910 to 1.4930. The water content has decreased to 7.5 to 9.5% byweight. The spinning solution is degassed, filtered and transferred tothe spinning vessel.

The modifying compounds for improving the suppleness must be misciblewith the cellulose/NMMO/water solution. The content of these compoundsis preferably 0.5 to 20% by weight, particularly preferably 1 to 15% byweight, in each case based on the weight of the cellulose. The compoundsmay be mixed homogeneously with the spinning solution at a temperatureof 85 to 105° C., preferably 90 to 100° C. Particularly suitablemodifying compounds are starch, starch derivatives and cellulosederivatives (in particular esters or ethers of the starch or cellulose),as well as sugar esters, and in addition hydrophilic naturally occurringpolymers (preferably alginic acid and alginates, chitosan andcarrageenan). Suitable compounds are also hydrophilic synthetic polymers(preferably vinyl alcohol, vinyl acetates and acrylates) and polymerswhich simultaneously possess hydrophilic and hydrophobic properties(preferably esters from a sugar-such as sucrose-and fatty acids, theesters having an HLB of 1 to 15; HLB=hydrophilic-lipophilic balance). Ifappropriate, fatty acids and salts thereof, for example stearic acid orcalcium stearate, waxes and paraffins are also suitable. Finally,polyvinylpyrrolidone, copolymers of vinylpyrrolidone and2-(dimethylamino)ethylmethacrylate, copolymers of methyl vinyl ether andmaleic anhydride or of methyl vinyl ether and maleic acid monoalkylester may also be used. The modifying compounds may also becrosslinkable, as is the case with polyethyleneimines. They also act asinternal (primary) plasticizers. Impregnation with secondaryplasticizers (such as glycerol) can frequently be even entirely omittedif the content of the modifying compounds in the food casings accordingto the invention is great enough (generally of the order of magnitude 8%by weight or more, based on the weight of the dry cellulose).Furthermore, they generally decrease the tendency of the cellulose tocrystallize.

The spinning solution is extruded through the annular die preferably ata temperature of 85 to 105° C., particularly preferably 90 to 95° C. Theannular gap is generally 0.1 to 2.0 mm wide, preferably 0.2 to 1.0 mm.The width here must be adapted to the warpage. “Warpage” is defined asthe quotient of the velocity on leaving the annular gap (exit velocity)and the velocity at which the extruded tube is taken off (take-offvelocity). The warpage is generally 3.0 to 0.10, preferably 2.0 to 0.2,particularly preferably 1 to 0.4. The exit velocity, depending on theconstruction of the plant, is 5 to 120 m/min, preferably 10 to 80 m/min.It is also determined by the caliber. On the extruded tube,advantageously, only a low tension is exerted in the longitudinaldirection, which is essentially due to its own weight.

The “air section”, i.e. the section between annular gap and surface ofthe spinning bath in which the blow molding takes place, is preferably 1to 50 cm, particularly preferably 2.5 to 20 cm. It is also dependent onthe diameter (“caliber”) of the tubular film after the blow molding. Incontrast to the abovementioned WO 95/07811 and EP-A 662 283, no measuresare required for additional cooling in the air section, and accordinglythey are also not provided. The extruded tube cools only a small amountin the air section. Otherwise, transverse stretching would scarcely bepossible. The blow molding is effected by compressed air or other gaseswhich pass into the interior of the tube through orifices in the diebody. Stretching in the transverse direction considerably increases thetransverse strength of the tube. Depending on warpage, the diameter ofthe blow-molded tube is up to 100% greater or up to 50% smaller,preferably up to 80% greater or up to 20% smaller, than immediatelyafter exiting the annular gap. Transverse stretching with a diametersimultaneously becoming smaller is obviously only possible if thewarpage is less than 1. Preferably, the diameter of the blow-molded tubeis 10 to 100% greater, particularly preferably 20 to 80% greater thanimmediately after exiting the annular gap.

If appropriate, the tube is conducted via a pipe, preferably a metalpipe. The diameter of this pipe can be selected between 30% greater and30% smaller than that of the annular gap. Precipitation liquid andsupport air are fed via this pipe.

After entering the spinning bath, the diameter of the tube decreases.Through appropriate apparatuses in the die body, the spinning bathsolution also passes into the interior of the cellulose tube. As aresult, the tube solidifies more rapidly; at the same time, the insidesare prevented from sticking together. The liquid level in the interiorof the tube should not be significantly higher or lower than that of thesurrounding spinning bath. The spinning bath itself is an aqueoussolution which comprises 5 to 50% by weight, preferably 8 to 20% byweight, of NMMO. The temperature of the spinning bath is in the rangefrom 0 to 50° C., preferably 2 to 20° C.

The depth of the spinning bath is determined by the caliber of thecellulose tube, its wall thickness and the desired residence time in thebath. Generally, the depth should be selected so that, on flattening thetube on the guide roll, the resulting edges are not damaged. In the caseof a tube of caliber 20, which, immediately after leaving the annulargap, has a wall thickness of 0.5 mm and passes through the bath at avelocity of 20 m per minute, the spinning bath has a depth of about 3 m.

For further solidification, the laid-flat tube then passes through stillmore NMMO-containing precipitation vats. The first precipitation vatcomprises approximately 10 to 20% by weight of NMMO. In the followingprecipitation vat, the NMMO content decreases. It has been found to befavorable to increase the temperature from one precipitation vat to thenext, up to about 60 to 70° C. in the last vat. The NMMO content in thetube is thus more greatly decreased.

This so-called “precipitation section” is followed by water-filled washvats, in which the last traces of NMMO are removed from the tube. Thetemperature of these baths is 15 to 70° C., preferably 40 to 60° C.Generally, a so-called plasticizer vat then follows. This comprises anaqueous solution of a plasticizer for cellulose. Suitable plasticizersare polyols and polyglycols, in particular glycerol. The aqueoussolution comprises 5 to 30% by weight, preferably 6 to 15% by weight, ofplasticizer. The temperature of the plasticizer solution isadvantageously 20 to 80° C., preferably 30 to 70° C. The glycerolcontent of the casing is then about 15 to 30% by weight, preferably 18to 23% by weight, in each case based on its total weight.

Thereafter, the tubes are conducted through a hot-air dryer in theinflated state. Expediently, drying is performed at decreasingtemperature (from about 150° C. at the inlet to about 80° C. at theoutlet of the dryer). An additional transverse orientation may beachieved, if appropriate, by appropriately increased internal pressureon drying. Otherwise, the tube is inflated on drying to the originalcaliber, in order to retain the degree of transverse orientation onceachieved. During drying, the swelling value decreases to 130 to 180%,preferably 140 to 170%, depending on drying conditions and glycerolcontent. The tube is then wetted until the water content is 8 to 20% byweight, preferably 16 to 18% by weight, in each case based on the totalweight of the tube. Then, using a pinch-roll pair, it can be laid flatand wound up.

Used aqueous NMMO solution may be purified by ion-exchange columns. Thewater can be taken off under reduced pressure until the NMMOconcentration has reached 60% by weight. This NMMO solution can then beused again for preparing the spinning solution. The NMMO is thusvirtually completely recovered.

Depending on caliber, the finished tubes, at a glycerol content of 20 to22% by weight and a water content of 8 to 10% by weight, in each casebased on the total weight of the tube, have a weight of 30 to 120 g/m²,preferably 35 and 80 g/m². The weight per unit area generally increaseswith increasing caliber. The bursting pressure is likewise dependent onthe caliber (small calibers have a higher bursting pressure). For a tubehaving a caliber of 16 mm, the bursting pressure is about 60 kPa, for acaliber of 30 mm about 40 kPa, at a caliber of 50 mm about 24 kPa and ata caliber of 140 mm about 15 kPa. The bursting pressure is measured ineach case here in the wet state.

The tubular casings according to the invention can, furthermore, beprovided on the inside and/or outside with an impregnation or coating,e.g. a liquid smoke impregnation or an “easy peel” internal preparation.The same obviously applies to flat films.

An essential advantage of the flat or tubular films according to theinvention is the uniform structure and thus uniform density which isachieved on precipitation. Films which are produced by the viscoseprocess, in contrast, have a density gradient (higher density on thesurface, lower in the interior).

The tubular films according to the invention are preferably used assausage casings, in particular as “peelable casing” in the production offrankfurters. In addition, they can also be used as membranes forvarious purposes, e.g. in hemodialysis. Finally, flat films can also beproduced by cutting open the tubes.

If the cellulose tubes are used as sausage casings, the stuffing calibercan correspond to the annular gap diameter or up to 120% above it.Preferably, the stuffing caliber is 10 to 80% above the annular gapdiameter.

The following examples serve for more detailed description of theinvention. Percentages therein are percentages by weight, unless statedotherwise. Flat width, weight of the casing and thickness of the casingwall were determined under standard conditions (55% relative humidity;23° C.).

EXAMPLE 1

510 g of ground wood cellulose (®Cellunier F from Rayonier) having amean degree of polymerization of 535 (determined by the Cuoxam method)was mashed in 5087 g of a 60% strength NMMO solution. The pH of the mashwas then adjusted to 11 by NaOH. Under reduced pressure, with stirringand heating, water was then distilled off with increasing temperature,until, at an NMMO content of 87.7%, based on the total weight of waterand NMMO, the monohydrate was present (recognizable by a refractiveindex of 1.4820). During this phase which lasted for approximately 4hours, the vacuum was kept at 10 to 16 mmHg. After stirring for afurther 2 to 3 hours, the cellulose was completely dissolved at about 85to 95° C. In order that relatively little water is evaporated, thevacuum was set to about 200 mmHg during this time. The refractive indexthen ranged from about 1.4910 to 1.4930, which corresponds to a watercontent of 7.5 to 9%.

The spinning solution prepared in this manner was extruded at atemperature of 90° C. through an annular die at a gap diameter of 20 mmand a gap width of 0.5 mm. The tube first passed through an air sectionabout 10 cm in length at a velocity of 20 m/min. In the course of thisit was transversely stretched by air fed internally. It then passedthrough a spinning bath section of 3 m. The spinning bath comprised a14% strength NMMO solution which was cooled to 5° C. A solution of thesame composition was also introduced into the tube interior (“innerbath”). The tube was then laid flat at a guide roll in the spinning vat.The tube had been stretched transversely to the extent that its flatwidth after leaving the spinning vat was 30 mm. The edges showed nodamage.

The tube then passed through 4 precipitation vats each having 8 guiderolls at the top and bottom, a bath depth of 1 m and an air section of 2m. At the end of the last vat water was introduced which was conductedin counter-current. At the outlet of the first vat, the NMMO content waskept in this manner at 12 to 16%. The temperature increased up to 60 to70° C. in the last vat. After passing through this precipitationsection, residues of NMMO were washed out of the tube in 4 washing vats.The temperature in these vats was likewise 60 to 70° C. Finally, thetube was conducted through a plasticizer vat which comprised a 10%strength glycerol solution having a temperature of 60° C.

At a swelling value of 290%, the finished tube absorbed 21% glycerol.The flat width on leaving the glycerol vat was still 20 mm. The tube wasthen dried with hot air between 2 pinch-roll pairs. The dryer had aplurality of zones of decreasing temperature. The zone at the inlet hada temperature of 120° C., and that at the outlet 80° C. Subsequently,the tube was wetted until its water content was 8 to 12% (based on theweight of cellulose) and was wound up. The bursting pressure of thistube was 52 kPa, its static extension was 20.5 mm, and its swellingvalue was 165%. It was then wetted to 16 to 18% and gathered in sections(“shirred to form shirred sticks”).

The shirred sticks were stuffed with sausage emulsion on an automaticstuffing machine (®FrankAMatic), scalded and smoked. Thereafter, thecasing was peeled by an automatic apparatus. In the scalding and smokingbehavior, this peelable skin was at least as good as one produced by theviscose process.

EXAMPLE 2

A spinning solution as described in Example 1 was extruded at atemperature of 90° C. through an annular die 45 mm in diameter and a gapwidth of 0.7 mm. At a velocity of 20 m/min, the tube formed in thismanner passed through an air section of 15 cm in order then to beimmersed in the spinning bath. In the air section it was stretchedtransversely, as described above, with compressed air. The spinning bathhad a depth of 3 m and was filled with a 12% strength aqueous NMMOsolution which had a temperature of 5° C. Spinning bath solution wascharged into the interior of the tube. On leaving the spinning vat, thetube had a flat width of 56 mm. Its swelling value was 302%. It was theninflated with reinforcing air, so that the flat width increased again to66 mm. Before being wound up, it was wetted until the water content was14 to 16%, based on the weight of the dry tube. The glycerol content was20% at a total weight of 56 g/m². Swelling value was determined at 158%.The soaked tube had a bursting pressure of 30 kPa. Its static extensionat an internal pressure of 15 kPa is 44 mm.

Sections of this tubular film each having a length of 50 m were shirredto form shirred sticks, which were then stuffed by an automatic stuffingmachine using fine Mettwurst emulsion to a stuffing caliber of 44 mm.The sausages were then matured in the usual way and smoked. The casingsaccording to the invention showed in this case properties at least asgood as cellulose casings produced by the viscose process.

EXAMPLE 3

The spinning solution described in Example 1 was extruded at atemperature of 90° C. through an annular die having a die gap diameterof 26 mm and a die gap width of 0.6 mm. In contrast to the two precedingexamples, the die body was joined to a 50 cm-long metal pipe over whichthe extruded tube was conducted. At the top end of the pipe there weresituated orifices for feeding compressed air (“support air”) requiredfor the transverse stretching and the spinning bath solution. The airsection between die gap and surface of the spinning bath was 2.5 cm. Thespinning bath was filled with a 3° C., 14% strength aqueous NMMOsolution. The spinning speed was 20 m/min. Sufficient support air wasfed so that the tube had a flat width of 40 mm on leaving the spinningvat. Further treatment was then performed as described in Example 1.After leaving the plasticizer vat, the flat width was still 28 mm. Theswelling value of the plasticized tube was 286%. It was then dried inthe inflated state, wetted to 12 to 16% water content and wound up on aroll. The finished tube had a weight of 44 g/m². It comprised 22%glycerol (based on its total weight) and a swelling value of 165%. Itsstatic extension at an internal pressure of 20 kPa was 25.8 mm, and itsbursting pressure (measured in the wet state) was 42 kPa.

The tubes were shirred in sections to form shirred sticks and werestuffed with sausage emulsion to a stuffing caliber of 26 mm on anautomatic stuffing machine. After scalding and smoking in theconventional manner, the casing was peeled off on an automatic plant andthe sausages were then packed in cans. The sausage casings according tothe invention complied with all requirements at least as well as thecasings produced by the viscose process.

EXAMPLE 4

The spinning solution prepared in accordance with Example 1, whichcomprised about 9% cellulose, was homogeneously mixed, with stirring,with 3%, based on the weight of the cellulose, of a sucrose monostearicacid/palmitic acid ester at a temperature of 95° C. It was then (at thesame temperature) extruded through an annular die having a diameter of20 mm and a gap width of 0.5 mm. At a velocity of 20 m/min, the tubefirst passed through a 10 cm long air section. In the course of this itwas pressurized internally with compressed air and thus transverselystretched. Further production steps were identical to those described inExample 1.

The casings gathered in sections (“shirred sticks”) were then placedonto the automatic stuffing machine and stuffed with sausage emulsion.After scalding and smoking, the casing was peeled off by an automaticapparatus. During scalding and smoking, the casing exhibited a behaviorlike the casings produced by the viscose process.

In the table below, the properties of a casing produced by theconventional viscose process termed “comparison”) are compared withthose of two casings according to the invention, the one (termed “A”)comprising no modifying compounds and having been produced according toExample 1, whereas the other (termed “Be”) comprising 3%, based on theweight of the cellulose, of the sugar ester mentioned in Example 4 andwas produced in accordance with this example.

Comparison A B Flat width* (mm) 29.2 26 28 Weight* (g/m²) 43.8 47.3 53.1Thickness* (μm) 40 55 45 Glycerol content** (%) 20.5 20.7 20.5 Swellingvalue (%) 154 165 169 Ultimate tensile strength***, 1.5 22.6 21.5longitudinal (N/mm²) Elongation at break***, 32 29.8 36.5 longitudinal(%) Change in length after wetting (%) longitudinal −0.9 −1.0 +1.0transverse −1.2 −1.0 −1.0 after drying again (%) longitudinal −2.5 −3.3−0.3 transverse −4.0 −10.5 −8.9 Bursting pressure*** (kPa) 48 52 50 *:measured under standard conditions **: based on the total weight ***: inthe wetted state

EXAMPLE 5

The spinning solution prepared in accordance with Example 1 was admixedwith 5%, based on the weight of the cellulose, of a copolymer of methylvinyl ether and maleic acid monobutyl ester (molar ratio 1:1) in theform of a 50% strength ethanolic solution and homogeneously mixed at atemperature of 95° C. The solution was then extruded through an annulardie having a diameter of 40 mm and a gap width of 0.7 mm. The tubepassed at a velocity of 20 m/min through an air section of 15 cm, withinwhich it was transversely stretched by compressed air. The furthermanufacturing steps were identical to those described in Example 1.

On leaving the spinning bath, the tube had a flat width of 66 mm, whichcorresponds to a diameter of 42 mm. On leaving the glycerol vat, theflat width was 56 mm, and the swelling value 302%. The tube was theninflated with compressed air, so that the flat width increased again to66 mm. Before being rolled up, it was wetted again until the watercontent was 14 to 16%. The glycerol content was 20%, based on the totalweight of the casing, and the swelling value was 158%. The wetted tubehad a bursting pressure of 30 kPa and a static extension of 44 mm at 15kPa.

Sections each 50 m in length were shirred to form shirred sticks whichwere then mounted on an automatic stuffing machine. The casing was thenmechanically stuffed with fine Mettwurst emulsion to a caliber of 44 mm,matured and smoked. The use properties corresponded to those of casingswhich had been produced by the viscose process.

The ultimate tensile strength (longitudinal) was 70%, the elongation atbreak 50%, above that of a casing likewise produced by the NMMO process,but without addition of the modifying compound. The shrinkage was 12%less than with the comparison material. The modified casing could bepeeled off more readily from meat sausage than the non-modified casing.

EXAMPLE 6

The spinning solution prepared in accordance with Example 1 was admixedwith 12%, based on the weight of the cellulose, of a copolymer ofvinylpyrrolidone and 2-(dimethylamino)ethyl methacrylate (molar ratio1:1) and homogeneously mixed at a temperature of 98° C. The solution wasthen extruded at this temperature through an annular die having adiameter of 26 mm and a gap width of 0.6 mm. At a velocity of 20 m/min,the tube passed through an air section of 50 mm, within which it wastransversely stretched by compressed air, so that on leaving thespinning vat it had a flat width of 40 mm. The further manufacturingsteps were identical to those described in Example 1. After leaving theplasticizer vat, the flat width was 36 mm, and the swelling value 286%.

The tube was then dried in the inflated state, wetted to 12 to 16% androlled up. It comprised 22% glycerol and then had a swelling value of165%. Its weight per square meter was 44 g. The bursting pressure (inthe wet state) was determined as 50 kPa. The static extension at 20 kPawas 25.8 mm. The ultimate tensile strength in the wet state was 60%, andthe elongation at break was 45%, over that of an unmodified comparisonmaterial.

The tubes were shirred in sections to form shirred sticks, which werestuffed with sausage emulsion on an automatic stuffing machine to astuffing caliber of 26 mm, scalded and smoked. The casing was thenpeeled off by an automatic apparatus and the sausages were packaged incans.

EXAMPLE 7

Example 6 was repeated with the difference that not 12% but 20% of thecopolymer of vinylpyrrolidone and 2-(dimethylamino)ethyl methacrylatewere mixed with the spinning solution. The solution was extruded throughan annular die having a diameter of 40 mm and a gap width of 0.6 mm. Ata velocity of 28 m/min, the tube passed through an air section of 12 cmin length, within which it was transversely stretched by compressed air.The further manufacturing steps were identical to those in Example 1.

On leaving the spinning vat, the tube had a flat width of 70 mm. Incontrast to the other examples, the tube here did not pass through asoftener vat, and therefore was free of glycerol. After drying it waswetted until the water content was 14 to 16% and then wound up. Theswelling value was 142%, the bursting pressure 42 kPa and the staticextension at 15 kPa was 44 mm. The shirred tube sections could bestuffed without problems on an automatic stuffing machine.

EXAMPLE 8

The spinning solution prepared in accordance with Example 1 was admixedwith 0.8%, based on the weight of the cellulose, polyvinylpyrrolidone K70 (mean M_(w): 200,000) and stirred at a temperature of 95° C. until ahomogeneous mixture had formed. The solution was then extruded throughan annular die having a diameter of 80 cm and a gap width of 0.7 mm. Ata velocity of 30 m/min, the tube passed through an air section 40 cm inlength, within which it was transversely stretched by compressed air.The further manufacturing steps were identical to those described inExample 1, but the spinning bath comprised a 10% strength NMMO solution.The glycerol vat comprised a 7%. glycerol solution.

The tube was dried in the inflated state in such a manner that the flatwidth of 130 cm was not changed. After wetting to 14 to 16% watercontent, the tube was wound up. It was then cut on one edge. The flatfilm obtained in this manner had a weight of 40 g/m², a thickness of 35μm, a glycerol content of 21%, a swelling value of 165%, an ultimatetensile strength of 20 N/mm² in the longitudinal direction and 16 N/mm²in the transverse direction, an elongation at break in the longitudinaldirection of 42% and in the transverse direction of 54%.

What is claimed is:
 1. A seamless tubular cellulose-based film obtainedby extruding a spinning solution containing cellulose-,N-methylmorpholine N-oxide-, and water through an annular die andtreating the tubular film in an N-methylmorpholine N-oxide-containingaqueous spinning bath, the film being transversely stretched by blowmolding in an air section between the annular gap and surface of thespinning bath, wherein the spinning solution comprises 0.2 to 50% byweight, based on the weight of the cellulose, of one or more modifyingcompounds which increase the suppleness, strength, clipping stabilityand shear stability of the tubular casing and which are miscible withsaid spinning solution, wherein the modifying compound comprises ahydrophilic naturally occurring polymer, a hydrophilic syntheticpolymer, a polymer which simultaneously possesses hydrophilic andhydrophobic properties, or a polyethyleneimine.
 2. A film as claimed inclaim 1, wherein the cellulose content in the spinning solution is 7 to15% by weight, based on the total weight of the spinning solution.
 3. Afilm as claimed in claim 1, wherein the cellulose has a mean degree ofpolymerization of 300 to
 700. 4. A film as claimed in of claim 1,wherein the spinning solution comprises 90.5 to 92.5% by weight of NMMOand 9.5 to 7.5% by weight of water, based on the total weight of thesolvent of the spinning solution.
 5. A film as claimed in claim 1,wherein the spinning solution comprises 0.2 to 50% by weight ofmodifying compounds, based on the weight of the cellulose.
 6. A film asclaimed in claim 1, wherein the modifying compound comprises starch, astarch derivative or cellulose derivative, a sugar ester, alginic acidor an alginate, chitosan, carrageenan, vinyl alcohol, vinyl acetate, anacrylate, an ester of a sugar and fatty acids, the ester having an HLBvalue of 1 to 15, a fatty acid or salt thereof, wax, paraffin,polyvinylpyrrolidone, a copolymer of vinylpyrrolidone and2-(dimethylamino)ethyl methacrylate, a copolymer of methyl vinyl etherand maleic anhydride, or a copolymer of methyl vinyl ether and maleicacid monoalkyl ester.
 7. A film as claimed in of claim 1, wherein thespinning solution is extruded at a temperature of 85 to 105° C., throughan annular die having a gap width of 0.1 to 2.0 mm.
 8. A film as claimedin of claim 1, wherein the distance between the annular gap and asurface of the spinning bath is 1 to 50 cm.
 9. A film as claimed inclaim 1, wherein the spinning bath is an aqueous solution whichcomprises 5 to 50% by weight of NMMO and has a temperature of 0 to 50°C.
 10. A film as claimed in claim 1, wherein the extruded tube,downstream of the spinning bath, further passes through a plurality ofprecipitation and washing baths.
 11. A film as claimed in claim 1, whichis treated with a plasticizer.
 12. A sausage casing comprising a film asclaimed in claim
 1. 13. A flat film obtained by cutting open the tubularfilm of claim 1 in the longitudinal direction.
 14. A film as claimed inclaim 1, wherein the cellulose content in the spinning solution is about9 to about 12% by weight, based on the total weight of the spinningsolution.
 15. A film as claimed in claim 1, wherein the cellulose has amean degree of polymerization of 400 to
 650. 16. A film as claimed inclaim 5, wherein the spinning solution comprises about 1 to about 15% byweight of the modifying compounds, based on the weight of the cellulose.17. A film as claimed in claim 1, wherein the spinning solution isextruded at a temperature of 90 to 95° C., through an annular die havinga gap width of 0.2 to 1.0 mm.
 18. A film as claimed in claim 1, whereinthe distance between the annular gap and a surface of the spinning bathis from about 2.5 to about 20 cm.